A papermaking process consists of the formation of a paper sheet from aqueous slurry of pulp and additives and then gradual removal of water from the wet paper. Water removal by itself is comprised of several stages. In the first part of the process, termed the wet end, water is removed by gravity, vacuum suction and then the pressing of wet paper by press rolls. In the later part of the dewatering process termed the dryer section, residual water is removed by heating and evaporating it off of heated surfaces.
When the paper web reaches the press section of a paper machine, the paper consistency is about 20-25%. In this section pressure is applied to the paper by a series of press rolls to expel water and make paper sheet smoother. Paper consistency rises to 40-50% after pressing. Upon this reduction of water content fibers come into close proximity to each other and the degree of association and bonding grows significantly. Fibers not only adhere to each other but also tend to adhere to roll surfaces creating a drag in the paper web flow. Surface tension and adhesion between paper and roll surfaces grows significantly. In addition, deposition of sticky materials like pitch, extractives, organic solids, inorganic fillers and fine fibers onto roll surfaces can also hinder paper web release from roll surfaces. These issues are especially significant with paper made from recycled or resin containing pulps.
Increased paper adhesion and deposition of contaminants onto the roll surfaces may affect the runnability of the web eventually causing disruptions or breaks in the process. To compensate for this increased adhesion of the paper it becomes necessary to pull with additional force or to “draw” the paper web as it is transferred onto the next section of paper machine. However, increasing the draw has its own consequences and may negatively impact paper quality or cause breaks. To avoid these undesirable effects, a number of treatments have been utilized. These include modifications in roll cover materials, mechanical removal of deposits by doctor blades and/or application of paper release agents.
A number of different chemistries have been applied and practiced to enhance roll release. Several applications describe compositions containing hydrophobic actives or emulsions. For example, U.S. Pat. No. 6,468,394 discloses application of wax emulsions onto roll surfaces, wherein said wax should have a melting point below 60° C. According to this method, the wax melts on the warm roll surfaces forming a hydrophobic film thereby facilitating paper release from the roll surface. The other application, U.S. Pat. No. 6,558,513 teaches a method of improving the release of paper webs from the surfaces of press rolls by applying non-aqueous, non-curing hydrocarbon compositions, in which the preferred materials are hydrocarbon polymers, polybutenes with preferred molecular weight to be in the range from 400 to 700.
A method described in U.S. Pat. No. 6,139,911 discloses improvement in release properties by application of additives in the form of dilute microemulsions. Active components are selected from the group of oils, waxes, water insoluble surfactants and polymers. The application of stable emulsions based on an alcohol, a fatty acid or oil, lecithin, and water soluble or water dispersible surfactant is described in WO1996/26997.
U.S. Pat. No. 6,723,207 discloses application of a blend of cationic water soluble polymer, non-ionic surfactant and anionic surfactant to the papermaking roll. The composition has an overall positive charge. The cationic polymer is preferably quaternary ammonium compound like poly-diallyldimethylammonium chloride.
The patent application US2009/0159229 discloses compositions applied to press roll for improvement of detachability of wet paper. The compositions of the actives applied onto press roll surface are based on functionalized polyoxyethylene-polyoxypropylene block polymers.
WO1997/11225 discloses the treatment of central rolls in the press section by aqueous enzyme solutions wherein at least one substance adheres to the surface of the roil and “improves the reliability of the moving element in the process of paper production”.
U.S. Pat. No. 6,051,108 discloses removing or preventing the buildup of deposits in papermaking wet press felts and on forming wires. The cleaning solutions contain at least one acidic cleaning compound and peracetic acid.
U.S. Pat. No. 4,704,776 discloses silicone oil, silicone plastic and fluoroplastic as release agents for paper machine press rolls. WO2008/063268 discloses preparation of linear or branched fluorinated polymers with at least one urea linkage. Polymers are designed for surface treatments including surface cleaning, textile treatments, stain release improvement and others.